What is the Importance of Test Plan?

Making Test Plan document has multiple benefits

Help people outside the test team such as developers, 
business managers, customers understand the details of testing.
Test Plan guides our thinking. 
It is like a rule book, which needs to be followed.
Important aspects like test estimation, test scope, 
Test Strategy are documented in Test Plan, 
so it can be reviewed by Management Team and 
re-used for other projects.

Test Plan Identifier;
References
Introduction
Test Items
Features To Be Tested
Features Not To Be Tested
Approach
Pass/Fail Criteria
Suspension Criteria
Test Deliverables
Testing Tasks
Environmental Needs
Responsibilities
Staffing and Training Needs
Schedule
Risks and Contingencies
Approvals